Patent GB-898467-A (GB)
Title: Web movement control apparatus
IPC: G06K13/28
Filed: 1958-12-30
Abstract: 898,467. Automatic tension control systems. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 30, 1958 [Dec. 30, 1957], No. 42090/58. Class 38 (4). [Also in Groups XVI and XXXVIII] A web winding and unwinding feeding control device includes a first open-ended column for receiving a variable length of web, a second open-ended column in which one wall comprises an electric conductor, a flexible conductor within the second column, and means for urging a length of flexible conductor against the wall which varies in response with the length of the loop. As shown, magnetic recording tape 9 is fed past a tape reading and recording head 13 by reversible vacuum drive capstans 12 and 14 driven by separate motors through electromagnetic clutches controlled selectively to determine the direction of tape feed. The capstans are braked by separate electromagnetic brakes. The tape 9 forms loops in vacuum columns 11 and 15 connected to vacuum pumps 33, 34. The length of loop in each column is sensed by a sensing tape 41 and 42 secured at their bottom ends to perforated metal partitions 25 and 26 respectively and at their top ends to springs 39 and 40. The tape 41, Fig. 4, comprises a plastic tape 44 with a conductive coating 43 on one side facing a perforated metal wall 35, Fig. 1, against which the tape is drawn by vacuum created by a lowvacuum pump 60. Tape 41 engaging the wall 25 constitutes a variable capacitance, the length of tape 41 in contact with the wall determining the capacitance, this length varying in response to the length of tape loop in the column 11, since the tape 41 is only drawn against the wall 25 beneath the tape loop. The variable capacitance, constituted by the tape 41 and wall 25 and a fixed capacitance form part of a bridge network 71, Fig. 5, connected to a high-frequency oscillator 72. The output of the bridge network is a high-frequency signal whose amplitude is proportional to the difference between the variable capacitance and the fixed capacitance. This signal is amplified by an amplifier 73 and fed to an integrating network 74 which converts high-frequency pulses into a voltage proportional to the length of tape in the column, such voltage being a minimum when the tape is at the top of the column. A differentiating network 75 connected to the network produces a signal which is proportional to the rate of change of the length of tape within the column 11, Fig. 1, no voltage being generated if tape is supplied from a reel 77 as fast as the capstan 12 withdraws tape from the column. A second integrating network 76, Fig. 5, integrates signals from the capstan to produce a signal proportional to the average direction and magnitude of the tape transported by the capstan. The outputs of the integrating, differentiating network, and a comparing network 77 (for adjusting the output to zero when the tape is at a normal position e.g. " A," Fig. 1, in the column 11) are connected to a mixer circuit 78, Fig. 5, which adds these outputs together, the magnitude of the resulting voltage signal determines the speed of rotation reel 7, Fig. 1, and the sign of this voltage determines the direction of rotation. The output of the mixer circuit 78 is fed to a servomotor 79, Fig. 5, having a reversible motor driving the reel 7, Fig. 1. A reel 17 is driven by a motor 20 controlled in the same manner as the motor 19. Thus, the length of tape loops in the columns 11 and 15 are maintained substantially constant during tape feeding past the head 13. Specifications 733,807 and 847,821 are referred to.